Cullin 3 (Cul3) has recently been implicated in a multitude of different processes, including the oxidative stress response, autophagy, tumorigenesis, and differentiation. To investigate the role of Cul3 in mammary gland development, we created a mouse model system using Cre-lox targeting where Cul3 is specifically deleted from the mammary gland. Such MMTV-Cre Cul3 mice examined at 2 and 3 months of age show delays and defects in mammary gland development. Mammary ductal trees from Cul3-deficient mammary glands exhibit delayed forward growth through the mammary fat pad, dilation of the ducts, and abnormal morphology of some of the epithelial structures within the gland. Additionally, terminal end buds are larger and less plentiful in MMTV-Cre Cul3 mammary glands, and there is significantly less primary and secondary branching compared to control animals. In contrast, by 6 months of age, the mammary ductal tree has grown to fill the entire mammary fat pad in glands lacking Cul3. However, distorted epithelial structures and dilated ducts persist. MMTV-Cre Cul3 mothers are able to nourish their litters, but the process of involution is slightly delayed in mammary glands lacking Cul3. Therefore, we conclude that while Cul3 is not essential for mammary gland function, Cul3 is required for the mammary gland to proceed normally through development.